In a study to be published in the January edition of the journal Epidemiology, researchers at the University of California-Davis’ M.I.N.D. Institute have determined that these factors account for only a fraction of the increase. “Younger ages at diagnosis, differential migration, changes in diagnostic criteria, and inclusion of milder cases do not fully explain the observed increases,” the study found.

Cal-Davis epidemiology professor Irva Hertz-Picciotto, who led the study, said in the study press release that more research should be performed on the effect of environmental factors. “Right now, about 10 to 20 times more research dollars are spent on studies of the genetic causes of autism than on environmental ones,” she said. “We need to even out the funding.”

Autism is a neurodevelopmental disorder that impairs a person’s ability to communicate and interact with others, and causes restricted and repetitive behavior. It is usually diagnosed in children between the ages of 18 months and three years old. Autism is the most serious of the three autism spectrum disorders (ASDs), which also includes Asperger syndrome and pervasive developmental disorder not otherwise specified (PDD-NOS).

Because there is a widespread increase in autism cases, many researchers believe that there must be a link to changes in the environment. Studies have shown a link between autism and prenatal exposure to thalidomide and valproic acid, though “the concordance rate is not 100%,” writes Michael Szpir in Environmental Health Perspectives, “which suggests that a genetic predisposition is necessary for chemical and microbial factors to act as triggers.”

There has been a highly publicized debate over the effect of vaccinations in causing autism, based on the fact that the early signs of autism are seen soon after a child receives the MMR vaccine. Though there is little scientific evidence showing a link between vaccinations and autism, the U.S. federal government lost a lawsuit in March in which a child’s autism was blamed on vaccinations.